I had to give my two points to X-Com.
X-Com (two points)
Close Combat
Freespace 2
Half-Life
Homeworld
Master of Orion 2
Portal
Sid Meier's Alpha Centauri
Starsiege: Tribes
Tie Fighter
X-Com: Enemy Unknown aka Ufo Defense.
I am already a broken record with this game so here is the short version. X-Com wonderfully layered so many gameplay systems that it shames most modern strategy RPGs. Two decades of Oscar Worthy character development has produced little that can replicate the attachment I had with my fragile field agents.
Freespace II.
The game included everything I ever wanted in a space sim. Then it introduced a sense of scale to combat that simply dwarfed everything that had come before it. Finally, it took its own bombastic plot and blew it up. Nothing I did really mattered. It was fantastic. It is fantastic.
Homeworld.
Star Wars: Rebellion was released less than a year prior and brilliantly demonstrated what can go wrong with a 3D RTS. In contrast Homeworld surmounted that hurdle with an easy to use interface that just worked. It also did a fantastic job with its BSG inspired plot. Every slowly unraveling mystery was accompanied with a proper
mood-setting track.
Sid Meier's Alpha Centauri.
There are two kinds of progress: the methodical experimentation and categorization which gradually extend the boundaries of knowledge, and the revolutionary leap of genius which redefines and transcends those boundaries. Acknowledging our debt to the former, we yearn, nonetheless, for the latter.
Academician Prokhor Zakharov, "Address to the Faculty"
Starsiege: Tribes.
The wide open spaces seamlessly leading into indoor environments were novel. The use of vehicles and custom character classes were supposed to bring about the first Battlefield game. However, in testament to the interplay between gamers and developers something else happened. The
methodical approach was discarded and a
fast-paced legend took its place. This game should have been the foundation for all competitive team-based multiplayer FPS. Unrivaled.
Master of Orion 2.
Largely the game I measure every 4x game against. There are a few that compare favorably, but not many. You can't have 4x space without a proper strings/synth soundtrack. You can't have 4xspace without letting me design my own ships. You can't have 4x space without without blowing up a few planets.
Portal.
The game is great, but that alone is not why I have it here. This was the first game I pre-loaded and played with friends 'opening night.' It was a special treat waiting for the stupid thing to unlock and then racing everybody to the end. It is something I have not been able to repeat since. I hope DD 'opening night' eventually becomes a bigger event for games.
Close Combat.
A handful of RTS games could go in this spot. Close Combat will get the nod from me because of all the unique qualities it brought together. Unit procurement and resourcing was handled in a strategic map. Units themselves had ammo and morale values that to this day are rarely given such impact. Line of sight rules were similarly integrated with great care and attention to detail. That tank may have a clear view into the city, but it can't be certain there are no units there. My head throbs every time someone claims Company of Heroes introduced this little RTS sub-genre.
Half-Life.
The start of this game is the start of a lot of trends. How many FPS games since have opened with an expository vehicle ride filled with ambient narrative? Half-Life also sparked the final refinement of the modding scene before it hit the big time. TF and RA have a claim to the title, but I still consider Counterstrike to be king of the mod era that started with Aliens TC.
Tie Fighter.
They finally let you play as the noble Empire fighting off treacherous insurgents. Wonderful gameplay that eventually leads to the overpowered Tie Defender and stupid Missile Boat. This was made in the days when a licensed product didn't have to be some dumbed-down drivel.
This list is a second draft. It's hard including two space sims on the list, but they were that good.A lack of new interesting space games has probably skewed my list. I am missing a CRPG, a sandbox, a graphic adventure, and a god game. Honorable mentions: Nearly everything on every list after mine, Doom, Quake, Starcraft, Sim City, Arcanum, Fallout, Independence War, X3, Blood Money, HoMM, Natural Selection, Shogun: TW, HoI, System Shock, Privateer, Outlaws, Oregon Trail
Descent...
So many hall-of-fame class games.